Add component test utilities with `waitFor` function

- Add `waitFor` util for easier async testing 

src/sidebar/components/test/util.js

+/**
+ * Wait for a condition to evaluate to a truthy value.
+ *
+ * @param {() => any} condition - Function that returns a truthy value when some condition is met
+ * @param {number} timeout - Max delay in milliseconds to wait
+ * @param {string} what - Description of condition that is being waited for
+ * @return {Promise<any>} - Result of the `condition` function
+ */
+export async function waitFor(
+  condition,
+  timeout = 10,
+  what = condition.toString()
+) {
+  const result = condition();
+  if (result) {
+    return result;
+  }
+
+  const start = Date.now();
+
+  return new Promise((resolve, reject) => {
+    const timer = setInterval(() => {
+      const result = condition();
+      if (result) {
+        clearTimeout(timer);
+        resolve(result);
+      }
+      if (Date.now() - start > timeout) {
+        clearTimeout(timer);
+        reject(new Error(`waitFor(${what}) failed after ${timeout} ms`));
+      }
+    });
+  });
+}